btf: Allow referencing an enum value to pull in enum definition #1274
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danobi
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Apr 17, 2020
BPF Type Format (BTF) is especially powerful for tracing tools like
bcc and bpftrace. It allows tools to know about kernel data structures
and layouts without having to parse headers. Headers are problematic
because (1) they do not always come installed on production / user
systems, (2) headers may not always describe the correct struct layout
due to compile time flags, and (3) can be tricky to parse [0][1].
As BTF becomes enabled on more systems [2], it becomes more desirable to
have BTF contain everything a tracing tool needs. BTF overhead is quite
minimal (~1.5MB) [3] so using BTF in lieu of parsing headers is very
attractive.
While BTF already contains almost everything tracing tools need (eg
structs, enums, unions, function signatures, etc.), it is missing a lot
of flags. The reason is that most flags are defined as preprocessor
macros and are thus invisible to the compiler when it generates debug
info.
However, there is a solution: we can convert macro flags into enums.
This would be quite a complicated and long running task so I'm hoping
this patch can start a discussion. I'm sure I haven't fully considered
the implications so hopefully we can discuss it.
This patch, when applied to a kernel with BTF, allows bpftrace to "see"
the flags [4]:
# bpftrace --btf -e 'BEGIN { printf("%d\n", S_IRWXG); }'
Attaching 1 probe...
56
^C
[0]: iovisor/bcc#2133
[1]: iovisor/bcc#2547
[2]: https://www.spinics.net/linux/fedora/fedora-kernel/msg07746.html
[3]: https://facebookmicrosites.github.io/bpf/blog/2018/11/14/btf-enhancement.html
[4]: bpftrace/bpftrace#1274
It's useful to be able to reference an enum value to have bpftrace pull in the enum definition from BTF. This is especially useful for anonymous enums which cannot be correctly referenced.
danobi
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nice result of that refactoring you did before, I don't see any problem |
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It's useful to be able to reference an enum value to have bpftrace pull
in the enum definition from BTF. This is especially useful for anonymous
enums which cannot be correctly referenced.